I just did an experiment with my 2016 Honda CRV. I experimented with how easy or difficult it would be to climb over the back seat and go into the cargo area, and to see how much space there would be once in the cargo area and crouched down to try to open the rear hatch from inside.
First, I could climb through the two rear headrests, and I measured the distance between them, at approximately 19.5 inches. But if Kiely used the back seats regularly with a lot of friends in the car, the middle head rest may have been up (in the picture above, the middle rear seat head rest is down), and in that case, it wouldn't have been easy to climb into the cargo area. And the mechanism to lower the middle headrest would be hard to figure out during a crisis like a car being submerged in water.
Second, seated in the cargo area, my head touched the roof of the car. I am 5'2" tall, so Kiely would have had to really crunch down in the back to fit, if she really tried to take refuge in the cargo area while the car was sinking nose first.
Finally, I don't think a body would float very far if the car flipped and a person was trapped in the cargo area. I think the body would be wedged in, so to speak, and the likelihood of a body floating out of that area seems unlikely to me in this small, tight space, with headrests blocking the path a body might naturally take to float out of this area.
Not sure if I have described this well so that people can understand.
EBM Slight edits for clarification